MCB Tripping Curves Explained — B, C and D Selection with Brand Cross-Reference
Standards explainer compiled by cBallast. 3-page PDF; the full content is on this page.
Compiled by
cBallast
Subject
Miniature circuit breakers
Document type
Standards explainer
Reference
2026 edition
Issued
2026-10-06
Pages
3
Format
Web page and PDF
What B, C and D curves actually trip at (3–5, 5–10 and 10–20 × In), how to choose for cable runs, motors and transformers, and the cross-reference between the big MCB families — ABB S200, Schneider Acti9 iC60 and Siemens 5SL — with breaking-capacity grades.
What the curve letter sets
Every MCB trips two ways: a thermal element handles sustained overload (the same for all curves), and a magnetic element fires instantly on short circuit. The curve letter fixes where the instant trip begins, as a multiple of rated current In:
Curve
Magnetic trip
Built for
Watch out
B
3–5 × In
Long cable runs, resistive loads, sockets in dwellings — trips on the smallest fault current
Nuisance-trips on inrush (motors, many LED drivers)
C
5–10 × In
The industrial default: mixed and moderately inductive loads
Needs more fault current at cable end to trip instantly — check loop impedance on long runs
D
10–20 × In
Transformers, welding sets, motor groups with heavy inrush
Demands the highest fault current for instant disconnection; verify earth-fault clearance times
Same-rating, different-curve breakers are not interchangeable in either direction without checking: a C16 swapped to B16 may nuisance-trip the machine; B16 to C16 may stop a long lighting radial from meeting disconnection times.
The big three families, side by side
ABB S200
Schneider Acti9 iC60
Siemens 5SL
Breaking grades
S201/S202/S203/S204 by poles; 6 kA standard, S200M 10 kA
iC60N 6 kA · iC60H 10 kA · iC60L higher at common ratings
Cross-brand replacement at the same rating, curve and breaking capacity is routine in general distribution — DIN-rail pitch and busbar comb systems are where the brands differ, so a mixed board keeps one maker per comb section. Coordination with upstream MCCBs is its own discipline: see the selectivity guide.
Five-second selection recipe
Resistive/lighting, long run → B at the cable's rating.
General industrial mix → C (the default for a reason).
Transformer, welding, grouped motors → D, then verify fault-loop current covers 10–20 × In.
Breaking capacity ≥ the board's prospective fault current — grade letter (N/H, 6/10 kA) before anything else.
Poles: switch every live conductor the system earths demand (1P+N and 3P+N variants exist across all three families).
Frequently asked questions
What is the difference between a B16 and C16 breaker?
Only the instant-trip threshold: B fires at 3–5 × 16 A, C at 5–10 × 16 A. Thermal overload behaviour is identical — the choice is inrush tolerance versus minimum fault current.
When should a D-curve MCB be used?
Where inrush regularly exceeds ten times rated current — transformers, welding equipment, motor groups — and the supply can still deliver the 10–20 × In needed for instant tripping.
Is an ABB S201-C16 equivalent to a Schneider iC60N C16?
Functionally yes at 6 kA: same rating, curve and class trip alike. Physically they differ in comb-busbar and accessory systems, so boards stay single-brand per busbar section.
What does the N in iC60N mean?
The breaking-capacity grade — 6 kA; H raises it to 10 kA and L beyond at common ratings. The curve letter and ampere rating follow separately.
Can I put a C-curve in place of a B-curve to stop nuisance tripping?
Only after checking the circuit's earth-fault loop: the C curve needs roughly double the fault current for instant disconnection, which long runs may not deliver.